Feature Overview

The dialer software in Cisco IOS prior to Release 12.2(8)T had no way to dial two different destinations with the same IP address. More specifically, in networks where a network access server (NAS) supports dialing clients with overlapping addresses, dial-out attempts would fail. The Asynchronous Line Monitoring feature feature solves this problem.

A VRF—virtual routing and forwarding instance—is a per-Virtual Private Network (VPN) routing information repository that defines the VPN membership of a customer site attached to the NAS. A VRF consists of an IP routing table, a derived Cisco Express Forwarding (CEF) table, a set of interfaces that use the forwarding table, and a set of rules and routing protocol parameters that control the information that is included in the routing table. A separate set of routing and CEF tables is maintained for each VPN customer.

The Multiprotocol Label Switching (MPLS) distribution protocol is a high-performance packet forwarding technology that integrates the performance and traffic management capabilities of data link layer switching with the scalability, flexibility, and performance of network-layer routing.

The Cisco IOS dialer software is "VRF-aware for an MPLS VPN," which means that it can distinguish between two destinations with the same IP address using information stored in the VRF. This capability is provided to the dialer software by two existing Cisco IOS commands, dialer map and ip route, which have been enhanced to include VRF information.

In previous Cisco IOS releases, the dialer software obtained the telephone number for dial-out based on the destination IP address configured in the dialer map command. Now, the enhanced dialer map command supplies the name of the VRF so that the telephone number to be dialed is based on the VRF name and the destination IP address. The VRF is identified based on the incoming interface of the packet, and is used with the destination IP address defined in the dialer map command to determine the telephone number to be dialed.

The ip route configuration command also includes the VRF information. When a packet arrives in an incoming interface that belongs to a particular VRF, only those ip route commands that correspond to that particular VRF are used to determine the destination interface.

Benefits

The Asynchronous Line Monitoring feature feature allows the dialer software to dial out in an MPLS-based VPN. The MPLS VPN model simplifies network routing. For example, rather than needing to manage routing over a complex virtual network backbone composed of many virtual circuits, an MPLS VPN user can employ the backbone of the service provider as the default route in communicating with all other VPN sites.

This default route capability allows several sites to transparently interconnect through the service provider network. One service provider network can support several different IP VPNs, each of which appears to its users as a separate, private network. Within a VPN, each site can send IP packets to any other site in the same VPN, because each VPN is associated with one or more VRFs. The VRF is a key element in the VPN technology, because it maintains the routing information that defines a customer VPN site.

For additional benefits and information about MPLS, VPNs, and VRF, refer to the documents listed in the "Related Documents" section.

Related Documents

Additional information about configuring networks that can take advantage of the Asynchronous Line Monitoring feature feature can be found in the following Cisco IOS documentation:

Supported Platforms

The Asynchronous Line Monitoring feature feature is supported on the following Cisco router and access server platforms:

•Cisco 1600 series

•Cisco 1720

•Cisco 1721

•Cisco 1750

•Cisco 1751

•Cisco 3600 series

•Cisco 3631

•Cisco 3725

•Cisco 3745

•Cisco 7200 series

•Cisco 7500 series

•Cisco 802

•Cisco 820

•Cisco 828

•Cisco uBR7200

Determining Platform Support Through Feature Navigator

Cisco IOS software is packaged in feature sets that support specific platforms. To get updated information regarding platform support for this feature, access Feature Navigator. Feature Navigator dynamically updates the list of supported platforms as new platform support is added for the feature.

Feature Navigator is a web-based tool that enables you to quickly determine which Cisco IOS software images support a specific set of features and which features are supported in a specific Cisco IOS image.

Feature Navigator is updated regularly when major Cisco IOS software releases and technology releases occur. For the most current information, go to the Feature Navigator home page at the following URL:

Configures a VRF and next hop address combination that points to the interface where the dialer software should make the connection.

These commands are only part of the required configuration and show how to map a VRF and next hop address combination to a dial string. Refer to the documents listed in the section "Related Documents" and the example in the "Configuration Example" section, for details on where to include these commands in the network configuration.

Verifying Asynchronous Line Monitoring feature

The verification process described in this section is based on the following configuration:

interface dialer 1

dialer map ip 10.0.0.1 vrf blue name peer1 peer1_number

ip route vrf blue 10.0.0.1 255.255.255.255 Dialer0

To verify correct operation of the Asynchronous Line Monitoring feature feature, on the customer edge NAS, use the ping command in EXEC mode to place a call to peer. The expected result is that the NAS successfully dials out to peer1. If the call fails, use the show adjacency EXEC command to check CEF adjacency table information.

Troubleshooting Tips

If you encounter problems with the Asynchronous Line Monitoring feature feature, use the following debug privileged EXEC commands on the NAS to help you determine where the problem lies:

•debug ppp authentication

•debug ppp negotiation

•debug aaa authentication

•debug aaa authorization

•debug radius

•debug dialer

Monitoring and Maintaining Asynchronous Line Monitoring feature

To monitor and maintain the Asynchronous Line Monitoring feature feature, use the following EXEC commands:

Command

Purpose

Router# show adjacency

Displays CEF adjacency table information.

Router# show ip route

Displays all static IP routes, or those installed using the AAA route download function.

Router# show users

Displays information about the active lines on the router.

Configuration Example

This section provides a configuration example of the Asynchronous Line Monitoring feature feature on the simple network topology shown in Figure 1.

Figure 1 CE-to-PE-to-Peer MPLS VPN Topology

Note The network addresses and telephone numbers used in the following configuration are examples only and will not work if tried in an actual network configuration.

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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.